So, last week my fuel gauge jumped up way past full while driving. It had just dropped below half a tank and when I checked next, it was way past full. I know that this means there's a short somewhere in the fuel sending unit, but the truck drove just fine until last night. I left work and noticed that my ammeter was way below where it should be. If left running for a minute or two, it would come u to the center of the dial, but if I turned on an accessory (lights, heater, etc) the gauge would drop way down again. So I know I have a huge power draw somewhere in my truck, regardless if the ignition is on or not. This morning I tried to turn my truck on and it just went "clikclikclikclik." I pulled the battery cables off to prevent my battery from being permanently killed and the positive cable was sparking as I took it off. I got 12.6 volts from the battery unhooked from the truck last night. Ideas?

So, last week my fuel gauge jumped up way past full while driving. It had just dropped below half a tank and when I checked next, it was way past full. I know that this means there's a short somewhere in the fuel sending unit, but the truck drove just fine until last night. I left work and noticed that my ammeter was way below where it should be. If left running for a minute or two, it would come u to the center of the dial, but if I turned on an accessory (lights, heater, etc) the gauge would drop way down again. So I know I have a huge power draw somewhere in my truck, regardless if the ignition is on or not. This morning I tried to turn my truck on and it just went "clikclikclikclik." I pulled the battery cables off to prevent my battery from being permanently killed and the positive cable was sparking as I took it off. I got 12.6 volts from the battery unhooked from the truck last night. Ideas?

The fuel gauge could be an open/corroded wire, a short, or a bad sender.

Your alternator brushes may be badly worn or stuck and causing the alternator not to charge properly. Could also be a big short, but something should have burned up. Could also be a bad connection.

The battery cable sparking could be because the dome light was on, the key was on, or anything was drawing power.

In other words, you need to do a bit more diagnosis to figure it out. There is no cut-and-dried answer.